## Problem
We recently added slow GetPage request logging. However, this
unintentionally included the flush time when logging (which we already
have separate logging for). It also logs at WARN level, which is a bit
aggressive since we see this fire quite frequently.
Follows https://github.com/neondatabase/neon/pull/10906.
## Summary of changes
* Only log the request execution time, not the flush time.
* Extract a `pagestream_dispatch_batched_message()` helper.
* Rename `warn_slow()` to `log_slow()` and downgrade to INFO.
6621be6b7b